Method and apparatus for finishing prisms



H. w. Moss! METHOD AND APPARATUS FOR FINISVHING PRISMS Nov. 28, 1944.

Filed April 3, 1942 IN VEN TOR.

Patented Nov.;28, 1944 METHOD AND APPARATUS FOR FINISHING PBISMS alley W. Mogey, Stamford, Conn., assignor to l The Perkin-Elmer Corporation,

Glenbrook,

Conn., a corporation of New York Application April 3, 1942 ,-Serial No. 437,519 4 Claims. (Ci. 51-284) This invention relates to precision optics, and more particularly tothe manufacture 'of prisms.

In the manufacture of various prisms it is important to, achieve high accuracy in the angle between two polished surfaces in the finished article, and it has been known in the manufacture of roof prisms to use a generally rectangular oblong bloclrhaving a parallel top and bottom and having its two long sides perpendicular to its top, placing the semi-finished roof prisms in. optical contact with said block with the unfinished roof surfaces substantially coextensive with the long bottom surfaces of the block, grinding and polishing the unfinished surfaces, and testing for any wedge formation (which indicates an inaccuracy) by comparing the position of images from the top and the bottom of the block as shown in an autocollimating telescope.

My invention has for an object the simplification and improvement of this procedure and of the block and of the telescope ensemble used therein.

Other objects and advantages will appear from the following description of my invention.

In the drawing Fig. 1 represents a diagrammatic perspective of the telescope and plate ensemble employed;

Fig. 2 represents a perspective of the block; Fig. 3 represents a cross section through the block taken along the line HIII[ of Fig. 2, look- Fig. 5 represents a vertical section taken along the line V-V of Fig. 4 looking in the direction of the arrows.

In the drawing l indicates generally an autocollimating telescope, of a construction well known in the art. 2 is a triangular fiat metal plate supported at its comers by adjustable setscrews 3, 3, 3. Conventional framework l4, ii to hold, respectively, the telescope I and the setscrews 3 is provided. 4 indicates a line of sight of the telescope, aimed at approximately the center of the plate 2. I have shown this line of sight as being straight, but for convenience in observation it-is frequently desirable to add mirrors or the like.

Afllxed on plate 2 and formingan equilateral trianglecentered on the line of,sight l are the three supporting steel balls 5, 5, 5.

Triangular block 6 is formed of glass having to within less than the margin of error permissible in the finished article. Such a margin of error in the perpendicularity of the sidestc the top of block mightwell be of the order of 1 second of arc. The sides and top are polished surfaces. will be successively ground and polished and will vary from. perpendicularity to the sides only by the error permitted in the last previous grinding of the work pieces.

Ground into the top center of the triangular block is a depression i0, and into this depression is placed a metallic cup ll, held in place with a wax l2 which is insoluble in alcohol. Centered in the cup is a depression l3 adapted to receive the end of a customary element for mechanical grinding and polishing i The triangular block as described is believed to be new, and a distinct advance over the oblong rectangular block heretofore used. The old block could not successfully be machine manipulated on account of its tendency to tilt, while if sufficient breadth were given to it to overcome such tendency its area in relation to the area of the prisms attached to it for grinding and polishing became so great as to involve a great deal of non-productive grinding and polishing of, the bottom block surface. Although a square block has this last disadvantage, it is possible to use such a block in practicing my method. Stability of the block, while keeping its size within efllcient limits, is assured by making it substantially as shown with its vertical dimension less than its horizontal dimensions.

The method I follow in theuse of the apparatus shown is also believed to be new, and a distinct advance over the method previously followed.

contacting surfaces being clean and of precision quality, they 'will adhere without the need for cementing, as shown in Figs. 4 and 5.

The fine grinding of this unit is then begun,

' using either hand or mechanical means or both.

Using a test. disc of optical glass having paralits three sides I, I, I perpendicular to its top 8 lel polished top and bottom surfaces placed'on As a result of its use the bottom 8.

the three steel balls 8, they are so adjusted by means of the set screws 3 as to establish a plane accurately p rpendicular to the line of sight I of the telescope I.

During theiine grinding of the unit it is wiped clean from time to time and is placed on the three steel balls I. The bottom surface, not beins polished, reflects. no image, but an image is formed from the top polished surface, and since the plane of the bottom surface has been established as perpendicular to the line of sight. any deviation of this image from the cross hairs of the telescope indicates a wedge formation, and indicates its direction and extent.

The unit is then, subject to additional fine.

grinding as may be necessary, and when, on subsequent testing, the image is at the cross-hairs, the angle of the second roof face is accurately perpendicular to the angle of the first roof face, to within the margin of error of the triangular block itself.

The unit is then polished, no previous polishings having been necessary for testing purposes,

as has been customary in the prior practice.

The roof prisms are then separated from the triangular block and cleaned, as finished articles. Although I have described this invention with special reference to roof prisms, it is applicable to other prisms where an exact angle between two plane surfaces is desired, such angle being established between the sides of the block and its top surface.

It should also be noted that I have described the use of my method with an autocollimating telescope, which for many purposes is most convenient. It is perfectly possible, however, and is within the scope of the method I have invented,

to employ one telescope to furnish a beam of parallel rays, to employ another observin telescope aimed at such beam but at an angle thereto, to establish a plane with the adjustable balls such as will cause the test disc vto reflect the image of such rays into the observing telescope, and then l0 1. In an apparatus for forming a-second fiat precision surface on a piece of optical material at a specified angle to a polished first fiat precision surface on said material, a block of optical material having a polished precision top'plane and a plurality of polished precision side planes symmetrically disposed with respect to the center of the block, each of which planes makes the specified angle to the top plane and is adapted for at tachment of said first flat surface by optical contact, the vertical dimension of said block being less than its horizontal dimensions.

2. Apparatus according to claim 1 in which the block is triangular and has three polished precision side planes.

3. In an apparatus for forming a second flat precision surface on a piece of optical material at a specified angle to a polished first fiat precision surface on said material, a block of optical material having a polished precision top plane and a plurality of polished precision side planes symmetrically disposed with respect to the center of the block, each of which planes makes the specified angle to the top plane and is adapted for attachment of said first fiat surface by optical contact, and means at the center of said block adapted for engagement with a grinding and polishing machine, the vertical dimension of said block being less than its horizontal dimensions.

4o 4. Apparatus according to claim 3 in which the attaching means comprises a cup at the center of the top surface of the block.

HALLEY W. MOGEY. 

